284 Journal of Agricultural Research voi. xiii. No. 5 



Another method found successful in preparing pure calcium hydrogen 

 arsenate consists in dissolving a commercial calcium arsenate, which 

 may be a mixture of the two calcium arsenates under discussion, in the 

 smallest quantity possible of either hydrochloric or acetic acid, as large 

 a volume of water as convenient being maintained. When evaporated 

 upon a hot plate, either amorphous powder or crystals separate out, 

 depending upon rapidity of evaporation. 



A simple laboratory method that also proved successful consists in 

 adding slowly a solution of calcium hydroxid [Ca(0H)2] to a clear 

 aqueous solution of arsenic acid, and in stirring vigorously meanwhile, 

 until the acid is about three-fourths neutraUzed. By slow evaporation 

 crystals of pure calcium hydrogen arsenate separate out. 



The chief precaution that must be observed in order to obtain the 

 pure salt is to maintain an excess of acid or H-ion in the original solu- 

 tions. This excess of acid, however, must be at a minimum, or the 

 precipitate will redissolve. 



In the preparation of tricalcium arsenate three methods suggest 

 themselves from a theoretical standpoint, as indicated by the following 

 equations : 



(i) When pure calcium hydrogen arsenate is acted upon by an alkali: 



3CaHAs04 + aNaOH-^CagCAsOJj + NajHAsO^ + 2H2O 



(2) When arsenic acid is completely neutralized with a solution of 

 calcium hvdroxid: 



2H3ASO, + 3Ca(OH)2^Ca3(As04)2 + 6H2O 



(3) When solutions of calcium chlorid and sodium arsenate are al- 

 lowed to react : 



sCaCl, + 2Na3As04->Ca3(As04)2 + 6NaCl 



In all cases cited, if the reaction is permitted to reach an equilibrium 

 and conditions are controlled as indicated, it is possible pure tricalcium 

 arsenate would result. However, the last proved to be the most prac- 

 tical, and concordant results were obtained. The perfected method is 

 as follows: Cold solutions of both calcium chlorid and sodium hydro- 

 gen arsenate were prepared and the former made just sufficiently 

 alkaline that no calcium hydroxid precipitated out. To the sodium- 

 hydrogen-arsenate solution just enough sodium hydroxid was added to 

 change completely the salt to sodium arsenate. (Special care must be 

 taken lest too much sodium hydroxid should be added, as this would 

 form calcium hydroxid when combined with the calcium-chlorid solu- 

 tion.) Both solutions were then filtered and the clear calcium-chlorid 

 solution was added to the sodium-arsenate solution. A heavy volum- 

 inous precipitate formed that settled very slowly. This was washed 

 by repeated centrifuging and decanting off the supernatant liquid each 



